The present invention is related to a direct drive scanning CRT (cathode ray tube) display system. More specifically, the present invention is related to protection circuitry for such a system which prevents the output drive device, which controls yoke current, from drawing excessive current.
In known scanning CRT display systems the turning on and off of an output drive device which controls horizontal deflection yoke current is accomplished in accordance with a horizontal sync (synchronization) input signal. During a trace cycle, an electron beam is horizontally swept across a CRT surface in accordance with defection yoke current to define one horizontal line of a visual picture, and during a retrace cycle the beam is repositioned. If the horizontal sync input signal is lost, or if some transitions in this signal are missing, proper operation of the display system so as to prevent excessive current from being drawn by the output control device is implemented by a phase locked loop. Essentially, in the event of a missing horizontal sync transition, a phase locked loop which is free running will supply an artificial horizontal sync transition. While these systems work satisfactorily at low scan rates, at high scan rates such as 64 kHz inherent noise in such a phase locked loop creates noticeable visual distortion (jitter) on the picture defined by the scanning electron beam whose position is varied in accordance with the deflection yoke current. In addition, typically no protection is provided to prevent turning on the power output drive device in case the flyback voltage provided by the horizontal deflection yoke has not sufficiently decayed by the next time the power output device is scheduled to be turned on.